The intestinal epithelium is home to specialized subsets of intraepithelial T lymphocytes (IEL) that provide essential regulatory and effector functions. Chemokines and chemokine receptors play pivotal roles in guiding the trafficking of T cells during their differentiation from precursors in lymphoid tissues and their subsequent migration to extralymphoid sites. Absence of the CC chemokine receptor 6 (CCR6) in mice leads to perturbations of mucosal immunity including an increase in the number of small intestinal IEL. IEL from CCR6 deficient mice display selective expansion of the alpha beta TCR IEL subsets expressing CD8alpha/alpha that can develop from precursors in intestinal cryptopatches through an extrathymic differentiation pathway. Preliminary studies using mutant mice with an EGFP knock in mutation at the CCR6 locus demonstrate that CCR6 is expressed on a majority of the IEL precursors found in cryptopatches, but is absent from mature IEL. The central objective of the proposed studies is to identify the mechanisms by which absence of CCR6 leads to dysregulated local T cell differentiation in the intestine. The differentiation potential of IEL precursors from CCR6 deficient and wild-type mice will be compared using a competitive adoptive transfer model in which both types of precursor cells are allowed to differentiate into mature IEL following transfer into mice that lack endogenous cryptopatches and intestinal IEL (CD132 null mice that lack the common gamma chain of the IL-2 receptor). The first specific aim is to determine the stage of IEL differentiation at which CCR6 deficient IEL precursors begin to show preferential expansion compared to wild-type IEL precursors. Functional properties (cytokine production, lytic activity, and proliferative rate) of IEL derived from CCR6 null or wild type precursors will also be compared. The second aim is to investigate the potential precursor-product relationship between CCR6+ and CCR6- c-kit+ intestinal lymphocyte precursors by comparing their relative maturity and differentiation potential. Mechanistic insights into the CCR6 dependent regulatory pathway that normally maintains homeostatic control over intestinal IEL may lead to identification of new types of chemokine receptor targeted pharmacological manipulations useful in the treatment of human inflammatory bowel disease.